This invention relates to electrostatic recording methods in which recording is carried out with a multi-stylus electrode assembly or the like, and more particularly to an electrostatic recording method of forming an electrostatic latent image with a multi-stylus electrode assembly on a recording medium the surface of which is preliminarily charged, in which the recording medium is charged uniformly.
An electrostatic recording method has been known in the art in which signal voltages are applied to an array of charging electrodes insulated electrically from one another to form an electrostatic latent image on a recording medium such as an electrostatic recording sheet, and the latent image thus formed is developed with toner powder or the like and is then fixed. Furthermore, Japanese Patent Application Laid-Open No. 6941/1972 has disclosed a method in which, after a recording sheet is uniformly charged in advance at a predetermined potential, the discharge electrodes of the aforementioned multi-needle electrode assembly are grounded according to an image signal to partially erase the charges on the recording medium in correspondence to the image, and the resultant image is developed with toner or the like and is then fixed.
The latter method in which the recording medium is uniformly charged in advance and is then discharged is disadvantageous in that it requires a number of drivers and is high in manufacturing cost, and in that it is difficult to uniformly discharge the recording medium. Therefore, the firstly described method in which signal voltages are applied to the charging electrodes is generally employed at the present time. A device for applying the signal voltages has been disclosed by Japanese Patent Application Publication No. 38531/1976.
In such a device, auxiliary electrodes are provided at certain intervals on one surface of a block-shaped multi-needle electrode array. Only the auxiliary electrodes required for printing are driven, so that electric discharge is caused between the charging electrodes and the auxiliary electrodes, by applying necessary voltage to generate electric charges, and the electric charges thus generated are held on the recording medium. The above-described electrode assembly will hereinafter be referred to as "a Gould type multi-needle electrode assembly".
Where the polarity of the charging electrodes is positive, it is necessary to apply more than +350 V to the charging electrodes and more than -350 V to the auxiliary electrodes; that is, it is necessary to apply more than 700 V in total across the charging electrodes and the auxiliary electrodes.
In the method in which positive signal voltages are applied to the charging electrodes as described above, if the dot density of the charging electrodes is increased, inter-stylus discharge is liable to occur. For instance in the case of a dot density on the order of 8 dots/mm, inter-stylus discharge is liable to take place with about +350 V or higher. This is a serious problem in the maintenance of the multi-stylus electrode assembly.
Further, there is no true electric charge in the background portion of the recording layer. Therefore, when an electrostatic latent image formed on the recording medium is developed, for instance, with toner, the toner is liable to stick onto the background portion, thus making the latter dirty.